One hundred-ninety-four fetuses with irregular heart rates were seen over a five-year period at a tertiary center for fetal cardiology. The median gestation at referral was 31 weeks, with a range from 19 to 41 weeks. Of these fetuses, 157 had extrasystoles of either atrial or ventricular origin. Blocked atrial ectopic beats had led to slow ventricular rates (80–110 beats per minute) in 37 fetuses. The structure of the heart was normal in all except two fetuses. Postnatal outcome was known for 165 of the fetuses. Of these, 157 (95%) had an uneventful antenatal and postnatal course. Tachyarrhythmias developed in eight fetuses (5%) in either the prenatal (n=4) or postnatal (n=4) period. Five of 37 fetuses with blocked atrial ectopic beats (13%) developed a tachyarrhythmia. No fetus developed hydrops, and all infants survived. All cases had required treatment with antiarrhythmic drugs. The occurrence of an irregular heart rhythm in the fetus, therefore, is not always benign. Fetuses with blocked atrial ectopic beats require particularly close monitoring.
